I got rid of my SIII's rotten tub and used a SIIa tub too (they're usually in better shape, less corrosion).
The earlier Rovers didn't have the square reverse lights that the III has. I never found them effective anyway, so instead of putting them back in, I just mounted a square Hella worklight, wired into the reverse switch instead. Much better light that way.
One thing you run into is the LHD/RHD differences, as to which side the tag light is mounted, the oval plate, etc.
The rear panels on my replacement tub weren't perfectly straight, either. So, I took a sheet of aluminum, cut it to be the right width to fit the height of the tub along the edge of the door-jamb. I then put it on a sheet break and bent a 90 along the edge, to wrap the corner into the door-jamb. Put it into place, then used a sharpie to mark the waist-curve of the tub's outer side. Used a nibbler to cut the curve, with the vertical bits done on the break. Then, I used body glue to glue the new panel to the outside. Then when the galvanized capping for the rear lights goes on the outside edge of the tub, you can't tell that it is there, unless you look "really" close inside the door jamb.
FWIW...
-L
'72 SIII SW 88"
'60 SII 88" RHD